Looping audio file playback

[riknoll]

This is a feature request for the new sample playback support. It would be great to have a looping playback mode for audio files so that we can "stretch" short sounds to create sustained notes.

In most samplers I've used, this is done by surfacing two parameters:

• loop start - defines the sample in the audio file at which to start looping
• loop end - defines the sample in the audio file where the loop ends. After reaching this sample, we loop back to the sample defined by "loop start"

When playing back the looping file, the audio plays normally until we hit the loop start and then looping begins. You usually don't want to start the loop at the beginning of the sample because you want to preserve the "attack" of the sound, so it's best to delay the looping until the sound reaches its sustained volume.

It's also important to make sure that loop start and loop end are both samples that represent zero-crossings in the waveform of the audio to avoid audio "pops" at the loop point (but i think it's probably up to the author to make sure of that).